A hydraulic motor system that is installed as a travel device in a hydraulic shovel, a road roller, or the like includes a hydraulic motor mechanism that rotates by hydraulic liquid pressure and a reduction gear that reduces the speed of rotation of the motor mechanism to drive a wheel (drum).
In a piston motor including such a reduction gear, the temperature of the reduction gear rises when continuously operating at high speed, and a casing which accommodates the motor mechanism is also heated due to the rising temperature of the reduction gear.
In the piston motor disclosed in JP2004-60508A, hydraulic oil that has leaked out from the motor mechanism (leak oil) flows into the casing, and the casing is cooled by the hydraulic oil.
In the piston motor disclosed in JP2006-161753A, a portion of the hydraulic oil that drives a capacity-varying mechanism flows into the casing, and the casing is cooled by the hydraulic oil.